The present invention relates to a device and a method of decreasing electric power consumption in a steering angle detecting system for a vehicle, and more particularly, relates to the system with an analog-type steering sensor which detects a steering angle and direction of the vehicle by an output voltage having a sine wave form.
Generally, the vehicle performing various controls such as anti-lock control responsive to a steering angle of a steering wheel requires a steering sensor for detecting rotational angle and direction of a steering shaft. The steering sensor is attached to a steering system 1 comprising a steering gear box 2 which is connected to knuckle arms 3 at both ends thereof through tie-rod ends 4, respectively, and a steering column 5, as shown in FIG. 1A. The column 5 is a hollow circular cylinder through which a steering shaft 50 is inserted, as shown in FIG. 1B. The steering shaft 50 is connected to the box 2 at one end thereof through a universal joint 6. The other end of the column 5 is connected to a steering wheel 7 through a column cover 8. The cover 8 is fixed to the vehicle body by a bracket 9 and fastening bolts (not shown).
The shaft 50 has a large diameter portion 51 connected with the wheel 7, a small diameter portion 52 connected with the joint 6, a middle diameter portion 53 located between both portions 51 and 52 for connecting them to each other. The steering sensor 10 comprises photo-interrupters 11 and 12 respectively located in positions 90 degrees apart from each other, a photo-interrupter 13 for detecting a neutral position of the steering wheel 7, and a steering disc 55 fixed on an outer surface of the middle diameter portion 53 of the shaft 50, as shown in FIG. 1B,. The photo-interrupters 11 to 13 include photo emitters 11a to 13a such as light emitting diodes, and photo receptors 11b to 13b each paired with the emitters 11a to 13a, respectively. The disc 55 is located between the emitters 11a to 13a and receptors 11b to 13b, and has a plurality of slits 56 formed radially and opposed to the emitters 11a and 12a and the receptors 11b and 12b at even radius intervals, so that each of the receptors 11b and 12b generates the sine wave voltage at one pitch of the slit. The disc 55 also has a neutral slit 57 opposed to the emitter 13a and receptor 13b for indicating the neutral position of the shaft 50. Accordingly, the disc 55 is a so-called encoded disc.
The aforementioned prior art uses an optical type rotary encoder having the encoded disc, the emitter and the receptor as the steering sensor. Another example of the prior art may use a magnetic type rotary encoder as the steering sensor 10. The magnetic type encoder has a steering disc or drum mounted on the steering shaft 50 on which N poles and S poles alternate at even intervals. The magnetic encoder has magnetic reluctance elements or Hall effect elements, which generate a voltage of sine waveform-like dependent on the rotation of the disc or drum. An example of the prior art is disclosed in Japanese Utility Model Laid-open No. 62-51214 (1987).
There are various problems when the aforementioned rotary encoder such as the optical type or the magnetic type is applied to the steering sensor for the vehicle.
First, as the above rotary encoder basically detects an amount of a relative rotational angle change of the steering shaft, it is necessary for detecting an absolute steering angle to set a zero angle as the neutral position and to calculate and store a change amount from the neutral position by a microcomputer. Accordingly, a neutral position mark is formed on the rotational disc 55 or the rotational drum, so that the neutral position is set.
However, as the steering shaft generally enables to rotate two to four rotations between full rightward and leftward steering operations, the neutral position mark is detected at least three times at a zero degree point (neutral point), and two points when the steering shaft is turned 360 degrees from the zero degree point in the right and left directions. Accordingly, though the sensor may continue to detect the absolute angle as described above while the power is in "ON" state once the stored absolute angle is lost by shutting the power of the microcomputer, it is impossible to detect the absolute angle thereafter. Even if the power is supplied to the microcomputer again after once shutting, the zero degree neutral point needs to be reset from three detection points of the neutral position mark by complicated operation.
Accordingly, the power continues supply to at least a memory storing the absolute angle even if the ignition switch is turned off at parking, so that the absolute angle is kept stored and the change of the steering angle at parking may be detected.
A second problem relates to the high resolution at a steering angle detection.
For example, in the optical type rotary encoder, the receptor 11b of the photo-interrupter receives one time of the change of the light and shade and converts it into output voltage having the sine wave form while the rotary disc rotates by one pitch of the slits. In the magnetic rotary encoder, the magnetic sensor detects one time of the change of the magnetic force and converts it into the output voltage of the sine wave form while the rotary drum rotates by one pitch of the N and S poles.
Then, the rotary disc or drum rotates by one pitch and generates an output signal of one wave form. The output signal is changed into pulse forms which are counted by the microcomputer to obtain the change amount of the steering angle by the usual detection method for the steering angle. Accordingly, the resolution of the steering angle detection is determined by the number of slits in the rotary disc or the number of magnetic poles of the rotary drum in the method.
However, the rotary disc or drum may not be made larger due to space restrictions, so that it is impossible to increase the slits or magnetic poles. Accordingly, it is impossible to make a resolution higher enough.
In order to solve the problem, there has been developed a steering angle detection method using an optical or magnetic rotary encoder for generating two analog signals which vary like sine wave with a phase discrepancy of 90 degrees to each other as the vibration of the steering angle. A microcomputer calculates a steering angle on the basis of two digital signals converted from the analog signals by an analog/digital converter. By the method, it is possible to make the resolution higher in spite of the number of slits or magnetic poles because of one-to-one correspondence between the analog signals and the steering angle.
However, as a plurality of cycles of the sine wave are output in a rotation range of the steering shaft, the above method has the problem that the absolute angle is not able to be detected if the reference position is not reset once the power source is shut off in the same manner as the pulse count type.
As described above, the method for directly calculating the steering angle from two sine wave signals by analog/digital converter, is effective as the steering angle detection method for the vehicle because of the increased resolution. However, the method makes the power consumption of the steering angle detection system large in comparison with the pulse count method, so that it is difficult to compensate for extended operation by a backup of the battery at the parking.